A theoretical model that integrates the comprehensive effects of surface roughness, plastic deformation, adhesion, material strength and crystal orientation is established to quantitatively calculate ...the frictional force of diamond sliding on diamond. The effectiveness and accuracy of the model are verified by various nanofrictional experiments using an atomic force microscopy tip and molecular dynamics simulations sliding along the different orientations on the {100} and {110} crystal planes. Moreover, the internal reasons why the frictional force of diamond sliding on diamond is anisotropic are analyzed to be caused by the amorphous defects, dislocations and von Mises shear strain occurring on the rubbed surface.
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•An accurate model was established to predict anisotropic friction force of diamond.•The amorphous defects influence the value of frictional force of diamond.•The anisotropy of diamond friction is related to the dislocation and shear strain.
The friction coefficient, an important parameter to evaluate the dynamic properties of friction pairs, has been widely used in macro engineering fields. However, it is probably inappropriate to ...characterize the tribological properties at the nanoscale due to the strong size effect, and the conventional formula cannot reveal its determinants owing to its oversimple form. Therefore, in the present work, a new formula is deduced to overcome these shortcomings. The established formula for the friction coefficient considers the adhesion and discloses the relationship between the friction coefficient and the material properties of diamond. It effectively suppresses the dependency of the friction coefficient on the load, although such a dependency cannot be eliminated completely. Therefore, another new formula, independent of the loading force, is derived. Interestingly, the results indicate that the size effect is invariably observed in the friction coefficients derived from the three formulas due to different accumulation effects of debris atoms, which is verified by molecular dynamics simulations.
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A dual‐motor servo mechanism is a high‐order and strong‐coupling system with unknown nonlinearity, which brings challenges to controller design to realize high‐performance tracking and ...synchronization. This article proposes a finite‐time command filtered control strategy to address this problem. In tracking control design, finite‐time command filters are adopted to obtain the derivatives of virtual controllers, and the “explosion of complexity” problem in backstepping is thus solved. An improved compensation system is designed to reduce filtering errors. The tracking control is developed based on the combination of finite‐time control and command filtered approach. Moreover, to deal with unknown nonlinearities and uncertainties, fast finite‐time extended state observers are developed to observe lumped disturbances for the load and motor, respectively. In synchronization control design, two opposite control actions are developed using the speed difference to force two motors to rotate synchronously. The finite‐time convergence of the tracking and synchronization errors is proved. The efficiency of the proposed control strategy is verified via experiments conducted on a dual‐motor servo turntable.
This paper proposes a finite‐time command filtered control strategy to address both the tracking control and the synchronization control problems. The lumped disturbances are observed by extended state observers, and the finite‐time control approach achieves fast response and high precision.
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•The dependency of diamond phase transition on the frictional force is explained.•An origin of directional graphitization in mechanical polishing of diamond is revealed.•Mechanisms of ...anisotropic material removal rate of diamond are disclosed in a new light.
Molecular dynamics simulation is employed to represent the mechanical polishing of diamond crystal, in which two diamond substrates with free surface of {100} and {110} faces are established. Firstly, the polishing direction related to variations of friction force on the both substrates are analyzed, and the non-diamond carbon atoms produced due to the phase transformation, mainly sp2 and amorphous sp3 hybridizations, in the surface layer are further extracted. Then, the distribution planes of ordered sp2 hybridizations are fitted, on which the length and angles of C–C bonds are also calculated. Finally, dependence of the length of dislocation lines and the distribution area of localized von Mises shear strain in the polishing direction are discussed. The results suggest that friction force decreases gradually when the polishing direction transiting from the ‘soft’ direction to the ‘hard’ direction, and the same variation tendency is also observed on the phase transformation, length of dislocation lines as well as distribution area of localized shear strain. More interestingly, the appearance of ordered sp2 structures, which can be considered as graphitization of diamond, has an obvious directionality. It always generates on the {100} crystal planes, regardless of polishing on the {100} and {110} faces.
Due to their high electromechanical coupling and energy density properties, ultrathin piezoelectric films have recently been intensively studied as key materials for the construction of miniaturized ...energy transducers, and in this paper we summarize the research progress. At the nanoscale, even a few atomic layers, ultrathin piezoelectric films have prominent shape anisotropic polarization, that is, in-plane polarization and out-of-plane polarization. In this review, we first introduce the in-plane and out-of-plane polarization mechanism, and then summarize the main ultrathin piezoelectric films studied at present. Secondly, we take perovskite, transition metal dichalcogenides, and Janus layers as examples to elaborate the existing scientific and engineering problems in the research of polarization, and their possible solutions. Finally, the application prospect of ultrathin piezoelectric films in miniaturized energy converters is summarized.
With the rapid development of photoelectric communication and other fields, the demand for high-precision aspheric mirrors has been increasing. Predicting dynamic cutting forces is vital in selecting ...machining parameters and also affects the surface quality of the machined surface. This study comprehensively considers the effects of different cutting parameters and workpiece shape parameters on dynamic cutting force. The actual width of cut, depth of cut, and shear angle are modelled while considering the effects of vibration. A dynamic cutting-force model considering the aforementioned factors is then established. Using experimental results, the model accurately predicts the average value of dynamic cutting force under different parameters and the range of fluctuation of dynamic cutting force, with a controlled relative error of about 15%. The influence of workpiece shape and workpiece radial size on dynamic cutting force is also considered. The experimental results show that the greater the surface slope, the more dramatic the dynamic cutting force fluctuations. This lays the foundation for subsequent writing on vibration suppression interpolation algorithms. The influence of the radius of the tool tip on dynamic cutting forces leads to the conclusion that to achieve the goal of reducing the fluctuation of cutting forces, diamond tools with different parameters should be selected for different feed rates. Finally, a new interpolation-point planning algorithm is used to optimize the position of interpolation points in the machining process. This proves the reliability and practicability of the optimization algorithm. The results of this study are of great significance to the processing of high-reflectivity spherical/aspheric surfaces.
For compound fault detection of high-speed rail vibration signals, which presents a difficult problem, an early fault diagnosis method of an improved empirical mode decomposition (EMD) algorithm ...based on quartic C2 Hermite interpolation is presented. First, the quartic C2 Hermite interpolation improved EMD algorithm is used to decompose the original signal, and the intrinsic mode function (IMF) components are obtained. Second, singular value decomposition for the IMF components is performed to determine the principal components of the signal. Then, the signal is reconstructed and the kurtosis and approximate entropy values are calculated as the eigenvalues of fault diagnosis. Finally, fault diagnosis is executed based on the support vector machine (SVM). This method is applied for the fault diagnosis of high-speed rails, and experimental results show that the method presented in this paper is superior to the traditional EMD algorithm and greatly improves the accuracy of fault diagnosis.
In this paper, we use a moment-based method to test the existence of the individual and time effects in unbalanced panel data models with time-invariant regressors. Based on the difference of two ...variance estimators of idiosyncratic errors, three test statistics are proposed. The test statistics for individual (time) effect is robust when the time (individual) effect exists, and is robust for the correlation between explanatory variables and individual or time effect. Additionally, they do not require prior distributional assumptions on the error term. The asymptotic properties of estimators and the test statistics are given in this paper. The Monte Carlo simulations show that the test statistics have good power in finite samples at various situations and a real example is studied for illustration.
Molecular dynamics simulations have become an important tool for the study of structures, dynamics, and functions of biomolecules. Time scales and force field accuracy are key factors for the ...reliability of these simulations. With the advancement of computational platforms and simulation technologies, all-atom simulations of proteins in explicitly represented aqueous solutions can reach as long as milliseconds. However, there are indications of minor force field flaws in literature. In this work we present our observations on force field accuracy under uncommon conditions. We performed molecular dynamics simulations of BBL refolding in aqueous solutions of acidic pH and high salt concentrations (up to 6 M) with the AMBER99SB-ILDN force field for a microsecond time scale. The reliability of the simulations relies on the accuracy of the physical models of protein, water, and ions. Our simulations show the same trend as experiments: higher salt concentration facilities refolding. However, we have observed the presence of β-sheet in the native helical region as well as α-helix and β-sheet in the native loop region. Some of the nonnative secondary structures are even more stable than native helices. Aside from the secondary structure issue under the uncommon conditions, we have found that the rigidity of glycine dihedral angles in the loop region leads to low root-mean-square deviations but large topological differences from the native structure. Whether this is due to a force field deficiency or not needs further investigations. Recently developed ion parameters exhibit evident liquid features even in the 6 M LiCl solution. However, cation–anion interactions in TIP3P water still seem too strong, leading to high fractions of contact ion pairs. We do not find any specific ion-binding motif, thus we conclude that the effect of salt is a nonspecific electrostatic screening in our simulations. Our observations on the AMBER force field performance under acidic conditions and high salt concentrations show that simulations under extreme conditions can provide informative tests of physical models.
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